JPH10227884A - Control rod assembly of fast reactor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent concentration of molten fuel in a hypothetical accident causing melting damage in fuel by providing an opening in control rod guide tubes. SOLUTION: In a control rod assembly 1, a control rod 8 is inserted inside a control rod guide tube 7, and in the control 8, neutron absorber rods 9 are bundled. On the side of the control rod guide tube 7, an opening 12 is provided, which is connected to a gap space 15 formed between wrapper tubes 16 of adjacent fuel assemblies 2. In case at the worst a core melt accident occurs, melting starts at the center of the adjacent fuel assemblies 2 and the wrapper tubes 16 melt, and the melt moves to the lower part touching the control rod guide tubes 1 and being cooled. In this time, the melt flows into the guide tubes 7 from the opening 12 passes from the guide tube lower part to an entrance nozzle 6 to be exhausted the core inlet plenum. By this, fission reaction is terminated without increase of reactivity and core anomaly event can be ended in early time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a control rod assembly for a fast reactor.

[0002]

2. Description of the Related Art In general, the core of a fast reactor (fast breeder reactor) is formed by collecting a large number of fuel assemblies in which fuel elements are arranged in a hexagonal trumpet tube and bundling them in a cylindrical shape. Further, a predetermined number of control rod assemblies for controlling the nuclear fission reaction of the fuel using the neutron absorbing material are arranged in the core fuel assembly arrangement. The control rod assembly is usually composed of control rods and control rod guide tubes (FIG. 6), and the control rods are suspended from a drive mechanism provided in an upper core mechanism just above the core, and from below the core. It is housed in a separately supported control rod guide tube. An upper spacer pad 3, an intermediate spacer pad 4, and a lower spacer pad 5 are provided on the outer periphery of the control rod guide tube, so that an interval between adjacent fuel assemblies in the core can be maintained. or,
An entrance nozzle 6 is provided below the control rod guide tube, and liquid sodium is supplied to the entrance nozzle 6 to cool the neutron absorber and the components of the control rod that are heated by irradiation of neutrons generated by nuclear reaction in the reactor core. Enter from 6,
The inside of the guide tube flows from bottom to top.

The control rod has a function of controlling a nuclear reaction in the reactor core by being driven up and down in a control rod guide tube by a control rod drive mechanism. That is, from the viewpoint of nuclear reaction in the reactor core, withdrawal of the control rod corresponds to progress of the nuclear reaction, and insertion corresponds to suppression of the reaction, and the starting, stopping, and power control of the furnace are performed by inserting and removing from the top.
In an emergency such as an earthquake, the control rod is quickly inserted from any inserted state to the full insertion position to stop the nuclear reaction of the core. Normally, boron carbide is used as a neutron absorbing substance used for a control rod.

[0004] As described above, even if an abnormality occurs during the operation of the reactor, the control rod is quickly inserted into the core and the reactor is stopped. However, from the viewpoint of further improving the safety of the reactor,
Measures to ensure safety even in the event of an abnormality occurring in the reactor, in which case control rods are not inserted or control rods are inserted, but control rods are inserted but unexpected performance may not be ensured. If it is settled, the safety of the reactor will be stronger. If the cooling performance of the reactor is not ensured, the reactor core will eventually be in a melting state, and the molten fuel will concentrate and the reactivity will increase. Even if such an emergency is assumed, if the safety and economic efficiency of the reactor can be ensured at the same time, the safety of the fast reactor will be strong.

[0005]

In the fast reactor, even if the fuel melts, the fission reaction stops if the fuel material is dispersed. If this is secured in the core structure,
The safety of the fast reactor will be further enhanced. In order to prevent the concentration of the molten fuel, the dispersion of the molten fuel needs to be performed before the formation of the core molten pool is extended to the entire core. As one of these measures, it is necessary to discharge the molten fuel from the core as soon as possible.

[0006] The melting of the fuel starts from a portion of the core where the decay heat density is high, that is, a portion where the power density during operation is high. The power density is high at the center of the core, and fuel melting starts from the center of the core. The downward direction of the core is effective as a flow path of such a molten substance. As the path in the downward direction of the core, the coolant inlet plenum below the fuel assembly or the coolant inlet plenum of the control rod assembly guide tube can be considered. Generally, at the time of such an accident, the lower part of the core is cooled by the coolant, and the melt in the core is cooled and solidified,
It is assumed that the flow path is closed.

[0007] Therefore, if a path for allowing molten fuel to flow downward without deteriorating the cooling characteristics and operating characteristics of the reactor core at the time of a virtual accident can be provided, the value in safety design is very large.

It is an object of the present invention to provide a fast reactor in which the concentration of the molten fuel is prevented even in a virtual accident where the fuel causes melting damage.

[0009]

In order to achieve the above object, the present invention provides a multi fuel assembly provided with nuclear fuel such as uranium and plutonium, and a multi fuel assembly provided with a neutron absorbing material such as boron carbide. In a core configuration of a fast reactor comprising a control rod assembly, a plurality of openings are provided on a side surface of a guide tube of at least a part of the control rods of the plurality of control rod assemblies. As a result, when a fuel melting accident is assumed, the molten fuel flows into the control rod guide pipe through this opening,
Further, the molten fuel passes through the inside of the control rod guide tube and passes through the entrance nozzle to the core coolant inlet plenum, whereby the molten fuel in the core is dispersed, and the reactivity is reduced by nuclear dilution.

Further, if the distance between the control rod guide tube and the adjacent fuel assembly is increased, even if the molten fuel solidifies on the surface of the guide tube, the molten fuel outside the surface solidified layer moves sufficiently. Molten fuel is discharged from the core through the opening of the guide tube, flowing to the lower part where possible.

In the core of the fast reactor constituted by the control rod assembly guide tube of the present invention, even if a core melting accident is assumed, before the entire core forms a molten pool, it passes through a hole formed in the control rod guide tube. The molten fuel flows inside the control rod guide tube,
By moving further downward and being discharged to the coolant core inlet plenum via the entrance nozzle, the reactor is shut down without increasing the reactivity.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. FIG. 1 is a schematic longitudinal sectional view of a control rod assembly 1 for a fast reactor according to a first embodiment of the present invention. In the control rod assembly 1 shown in FIG. 1, a control rod 8 is inserted inside a control rod guide tube 7, and a neutron absorber rod 9 is bundled in the control rod. A dash ram 10 is provided at the lower end of the control rod 8, and is formed so as to mesh with a dash pot 11 installed at a lower portion inside the control rod guide tube 7. It is designed to reduce the falling energy when inserted. At the lower end of the control rod guide tube 7 is an entrance nozzle 6 which is an inlet for coolant sodium.
The sodium 13 rises inside the control rod guide tube 7 and flows so as to cool the control rod 8 which is heated by irradiation of neutrons generated by nuclear reaction in the core. An upper spacer pad 3, an intermediate spacer pad 4, and a lower spacer pad 5 are provided outside the control rod guide tube 7, so that a space between adjacent fuel assemblies 2 in the core can be maintained. Here, between the control rod guide tube 7 and the wrapper tube 16 of the adjacent fuel assembly 2, upper and lower intermediate spacer pads 4 and lower spacer pads 5 are provided.
And a sodium region 15 closed by the control rod guide tube 7 and connected to the control rod guide tube side through an opening 12 provided on a side surface of the control rod guide tube 7.

FIG. 2 shows the outer shape of the control rod guide tube in the first embodiment. The upper spacer pad, the intermediate spacer pad, and the lower spacer pad are provided outside the control rod guide tube 7 in the same manner as the conventional example shown in FIG. 6, but between the intermediate spacer pad 4 and the lower spacer pad 5. The duct has an opening 12 on the side surface and is connected to the inside of the entrance nozzle 6. FIG. 3 shows a structure of a region including the opening of the control rod guide tube. Sodium that has flowed in from the entrance nozzle 6 at the lower end of the control rod guide pipe 7 passes through the opening 14 at the lower part of the guide pipe and rises in the gap between the inside of the control rod guide pipe and the dash pot. On the other hand, an opening 12 is provided on a side surface of the control rod guide tube, and is connected to a gap space formed between the control rod guide tube and a flapper tube of an adjacent fuel assembly. Since the position of the opening is less than the axial position of the neutron absorber (heating element) when the control rod is fully inserted, the control rod can be sufficiently cooled.

In the event that a core melting accident occurs, melting starts from the center of the height of the adjacent fuel assembly,
The fuel assembly wrapper tube also melts and moves downward along the surface of the guide tube while the melt touches the surface of the control rod guide tube and is cooled. During this time, the melt flows into the inside of the guide tube from the hole provided in the guide tube, moves down the inside of the guide tube, and further moves from the lower portion of the guide tube toward the entrance nozzle. If the gap between the wrapper tube and the control rod guide tube in the adjacent assembly is sufficient, the melt flows into the inside of the guide tube from the opening provided on the lower side of the control rod guide tube, and cools the core through the entrance nozzle. Because of the flow toward the material inlet plenum, the discharge of the molten fuel from the core is further facilitated.

In order for the molten fuel to be easily discharged out of the core through the gap between the fuel assembly wrapper tube and the control rod guide tube, it is confirmed that there is no blockage due to the re-solidification of the molten fuel in the gap. There is a need. Cheung, Baker, described in Owen C. Jones, Jr., Nuclear, Reactor, Safety, Heat Transfer, pp. 565-572, describes whether the melt causes blockage in the gap.
And models evaluated theoretically and experimentally by Eqstein, Yim, Cheung et al. In the present model, the solidified fuel crust is gradually formed in the circular pipe from the inlet of the circular pipe, and if the length of the circular pipe at that time is longer than a certain length, it is blocked by the solidified fuel in the circular pipe. At that time, a correlation equation was obtained from the pressure balance and the heat transfer balance as the penetration distance Xp of the melt in the circular tube.

[0016]

(Equation 1)

Assuming a pressure difference ΔP = 1 kg / cm ² and a guide tube temperature of 700 K, a good result can be obtained if the diameter of the circular tube is 1.2 to 1.3 cm in order to obtain an intrusion distance of about 2 m. .
Therefore, even if the gap between the aggregates is considered in terms of the equivalent diameter, a gap of 1.2 cm or more will be required. In this way, if a sufficient gap is provided between the control rod guide tube and the fuel assembly, even if the fuel melts, the melt easily moves to the position of the lower spacer pad of the assembly. If there is an opening through which the molten fuel can enter inside the control rod guide tube, the molten fuel can be more easily discharged from the reactor core.

FIG. 4 shows a second embodiment in which the gap between the adjacent fuel assembly wrapper tube and the control rod guide tube is increased. In this embodiment, in the event of a fuel melting accident, the molten fuel moves downward without clogging the gap between the fuel assembly and the control rod guide tube, and eventually melts the lower control rod guide tube wall. Then, the molten fuel is discharged out of the core via the entrance nozzle.

In the third embodiment shown in FIG. 5, the gap between the adjacent fuel assembly wrapper tube and the control rod guide tube is increased, and an opening is provided on the side of the control rod guide tube immediately above the lower spacer pad. By disposing the molten fuel at the same height as the opening leading to the gap, the molten fuel can be more effectively discharged from the reactor core.

[0020]

According to the present invention, even if a core melting accident occurs in a fast reactor, the molten fuel is discharged out of the core, so that the nuclear fission reaction stops and abnormal events in the core are terminated early. I can do it.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a control rod assembly according to a first embodiment of the present invention.

FIG. 2 is a perspective view of a control rod guide tube according to the first embodiment.

FIG. 3 is a partial explanatory view of a control rod guide tube according to the first embodiment.

FIG. 4 is a longitudinal sectional view of a control rod according to a second embodiment.

FIG. 5 is a longitudinal sectional view of a control rod according to a third embodiment.

FIG. 6 is a perspective view of a conventional control rod guide tube.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 control rod assembly 2 fuel assembly 3 upper spacer pad 4 intermediate spacer pad 5 lower spacer pad 6 entrance nozzle 7 control rod guide tube 8
... control rod, 9 ... neutron absorber rod, 10 ... dash ram,
11 dash pot, 12 side opening of control rod guide tube, 13 sodium coolant, 15 gap space, 1
6 ... Trumpet tube.

Claims (5)

[Claims] 1. A control rod assembly for a fast reactor, wherein the control rod guide tube has an opening in a control rod assembly having a control rod in the control rod guide tube. 2. The control rod according to claim 1, wherein the control rod is made of a neutron absorber, and the position of the opening of the control rod guide tube is determined from the axial position of the neutron absorber when the control rod loaded in the core is fully inserted. The control rod assembly of the fast reactor below. 3. The control rod assembly of a fast reactor according to claim 1, wherein one of the openings formed in the control rod guide tube is located immediately above a lower spacer pad. 4. A control rod assembly according to claim 3, wherein said control rod assembly is loaded in a reactor core and adjacent to a side surface of said control rod guide tube between a lower spacer pad and an intermediate spacer pad outside said control rod guide tube. A control rod assembly for a fast reactor having a distance of 1.2 cm or more between the fuel assembly and a side surface of the fuel assembly. 5. A control rod assembly loaded in a core,
A high-speed structure characterized by having a gap of at least 1.2 cm between the side surface of the control rod guide tube between the lower spacer pad and the intermediate spacer pad outside the control rod guide tube and the side surface of the adjacent fuel assembly. Furnace control rod assembly.